Optical phase modulators are used to control phase of optical signals in interferometers and other devices where optical phase is translated into signal power, for example in Mach-Zehnder modulators used in optical communications and Sagnac interferometers used in optical gyroscopes. Functioning of most optical phase modulators is based on so-called electro-optic effect, wherein refractive index of an electro-optic material depends on an external electric field. The electric field is usually created by applying a voltage to a pair of electrodes disposed on two sides of the electro-optic material. Light propagating in the electro-optic material undergoes a phase shift due to a change of the refractive index caused in the material by the voltage applied to the electrodes. To lower the operating voltage, an optical waveguide having a width of only a few microns is formed in the electro-optic material, and the electrodes for applying the electric field are placed in close proximity to the waveguide.
Referring to FIG. 1A, a prior-art Mach-Zehnder modulator 10 is shown as an example. The Mach-Zehnder modulator 10 includes an input port 11A, an input spatial filter waveguide 11B, a Y-splitter waveguide 12, branch waveguides 13 and 14, a Y-combiner waveguide 15, an output spatial filter waveguide 16B, and an output port 16A formed in a X-cut lithium niobate substrate 17 using an Annealed Proton Exchange (APE) method. The APE waveguides 11B, 12-15, and 16B are shown with thick solid lines. Electrodes 18 and 19 are used to create electric fields of opposing polarity in the branch waveguides 13 and 14. The branch waveguides 13 and 14, in conjunction with the electrodes 18 and 19, form two phase modulators creating optical phase delays of opposite sign, for doubling the phase delay effect. When a voltage is applied between the electrodes 18 and 19, the optical phase of light propagating in the branch waveguides 13 and 14 is modulated in a push-pull fashion. The output light power depends on the optical phase difference in the branch waveguides 13 and 14 and, therefore, depends on the voltage between the electrodes 18 and 19. Thus, the Mach-Zehnder modulator 10 can be used for modulating light or attenuating light in a controllable manner.
Referring to FIG. 1B, a prior-art Y-fed Balanced Bridge Modulator (YBBM) 20 is similar to the Mach-Zehnder modulator 10 of FIG. 1A, with a directional coupler 25 in place of the output Y-combiner 15. The directional coupler 25 is coupled through output spatial filter waveguides 21B and 22B to output ports 21A and 22A, respectively. Varying the relative optical phase of light propagating in the branch waveguides 13 and 14 results in controllable redistribution of light between the output ports 21A and 22A of the YBBM 20, which enables its use as a voltage-controlled variable splitter or routing switch.
Turning now to FIG. 1C, a prior-art Y-branch Dual Phase Modulator (YBDPM) 28 is similar to the YBBM 20 of FIG. 1B. No output coupler is used in the YBDPM 28, the phase modulated signals being directed by the branch waveguides 13 and 14 to the output spatial filter waveguides 21B, 22B and further to the output ports 21A, 22A, respectively. Electrode pairs 23-24 and 26-27 are used to change the optical phase of the light propagating in the branch waveguides 13 and 14. The top waveguide 13 and the top electrode pair 23-24 form a top optical phase modulator, and the bottom waveguide 14 and the bottom electrode pair 26-27 form a bottom optical phase modulator. The YBDPM 28 can be used in optical gyroscopes, by connecting a looped polarization-maintaining optical fiber to the output ports 21A and 22A. Applications of YBDPM for sensing rotation are described by S. Ezekiel and H. J. Arditty in Fiber-Optic Rotation Sensors, Springer-Verlag, Berlin, 1982, pp. 23, 52-81, 102-110, 124-135.
The APE waveguides 11B, 12-15, 16B, 21B, and 22B, shown in FIGS. 1A to 1C in thick solid lines, guide light of only one polarization and thus act as highly efficient polarizers. Polarizing property of APE optical waveguides is desirable in many applications.
Unfortunately, applications of variable splitters or attenuators where the splitting or attenuating ratio needs to be maintained constant over extended periods of time are hindered by time dependence of optical phase delay generated in the APE/lithium niobate waveguides 13, 14 upon prolonged application of DC voltage to the electrodes 18-19, 23-24, or 26-27. Referring to FIG. 2, the optical phase delay in degrees is plotted against time in minutes for the top optical phase modulator of the APE Mach-Zehnder modulator 10 of FIG. 1A, including the APE waveguide 13 and the electrodes 18 and 19 adjacent thereto. In FIG. 2, at the time of 0 minutes, a voltage V is applied between the electrodes 18 and 19. At the time of 10 minutes, the voltage V is reversed to −V. One can see that the optical phase does not stay constant but relaxes as a matter of minutes. The optical phase delay relaxes from −218 degrees to −188 degrees and from −125 degrees to −155 degrees in ten minutes. This drift is typical for modulators having APE waveguides formed in lithium niobate substrates.
Another known drawback of optical modulators having APE waveguides formed in lithium niobate substrates is associated with drift of electro-optical response in vacuum, due to instability of APE waveguides in vacuum. In US Patent Application Publication US2007/0116421, Hendry et al. passivated the waveguide surface to reduce vacuum sensitivity of APE waveguides. In US Patent Application Publication US2009/0219545, Feth partially replaced APE waveguides with waveguides obtained by titanium diffusion, herein termed “Ti diffusion waveguides”. Ti diffusion waveguides do not exhibit a significant drift of the electro-optical response in vacuum.
Referring to FIG. 3A, a prior-art Mach-Zehnder modulator 30 has an input port 31A, an output port 36A, and Ti diffusion waveguides 31B, 32-35, 36B in place of the APE waveguides 11B, 12-15, 16B, respectively, of the Mach-Zehnder optical modulator 10 of FIG. 1A. The Ti diffusion waveguides 31B, 32-35, 36B are shown in FIGS. 3A and 3B in thick dotted lines. The Ti diffusion waveguides 31B, 32-35, 36B are not sensitive to vacuum; however they guide light of both polarizations, not just one polarization. To preserve the polarization selection property in the Ti diffusion Mach-Zehnder modulator 30, a waveguide portion 38 coupled to the input port 31A was formed using an APE method. The APE portion 38 is “stitched” to the Ti diffusion spatial filter waveguide 31B at a stitching location 37.
Referring now to FIG. 3B, a prior-art YBDPM 38 is similar to the YBDPM 28 of FIG. 1C, the difference being that the branch waveguides 33 and 34 are Ti diffusion waveguides stitched to the APE Y-splitter 12 and to the output spatial filter waveguides 21B, 22B at the stitching locations 37. Ti diffusion branch waveguides 33 and 34 make the YBDPM 28 much less sensitive to vacuum.
Since APE and Ti diffusion waveguides are formed at different temperatures (approximately 300 to 400° C. and 1000 to 1050° C., respectively), they can be formed in different process steps, starting with forming Ti diffusion waveguides at approximately 1000 to 1050° C., and then forming APE waveguides at approximately 300 to 400° C. This process has been disclosed in U.S. Pat. No. 5,982,964 by Marx et al. For both the Mach-Zehnder modulator 30 FIG. 3A and the YBDPM 38 of FIG. 3B, the alignment of APE and Ti diffusion waveguides at the stitching locations 37 is ensured by careful alignment of photolithographic masks used in the APE and Ti processes to manufacture the Mach-Zehnder modulator 30 and the YBDPM 38. For the APE waveguides 38, 11B, 12, 21B, and 22B, a photolithographic mask is used to create narrow openings in a layer of deposited metal, such as aluminum (Al) or titanium (Ti). The patterned metal then functions as a proton exchange (PE) mask. The PE step is followed by an annealing step, for diffusing the protons deeper into the substrate 17. For the Ti waveguides 31B, 32 to 35, and 36B, a photolithographic mask is used to create a pattern of narrow stripes in a deposited layer of Ti, which then diffuse into the substrate in a diffusion furnace, creating the Ti waveguides. There are no intentional offsets or gaps between Ti and APE waveguides at the stitching locations 37.
Turning to FIG. 4, the optical phase delay in degrees is plotted against time in minutes for the top optical phase modulator of the Ti diffusion Mach-Zehnder modulator 30 of FIG. 3A having the Ti diffusion waveguide 33 between the adjacent electrodes 18 and 19. In FIG. 4, at time of 0 minutes, the voltage V is applied between the electrodes 18 and 19. At time of 10 minutes, the voltage V is reversed to −V. One can see that the optical phase does not stay constant but increases from −200 degrees to −225 degrees and from −165 degrees to −140 degrees in 10-minute time intervals. Thus, Ti diffusion waveguide-based Mach-Zehnder modulator 30, although insensitive to vacuum, also exhibits drift of optical phase upon application of a constant voltage. The time drift of optical phase delay generated by the APE and Ti diffusion waveguide-based modulators 10 and 30 and YBBM 28 and 38, respectively, is highly detrimental and limits areas of their applications.
It is a goal of the present invention to provide a stable optical phase modulator, in which both the vacuum sensitivity and the time drift of the generated optical phase difference would be substantially reduced.